On the desktop - no benefit. New Metro UI is the biggest change and it's primarily a touch-screen friendly interface suitable for tables or smartphones. Not at all for a "working" desktop where you might want to do more advanced tasks such as using a word processor or a spreadsheet. And since Microsoft's plan to compete in the smart phone market is to first surpass Blackberry mostly by waiting for RIM's demise as BlackBerry use hits 0, Metro is irrelevant before even getting out the door.
On the server - a lot of big benefits and well worth the upgrade. Windows Server 8 looks to be a big step forward from the late 20th century and into the early part of the 21st century. This PC Mag article from the fall gives a pretty good breakdown. I won't re-hash the author's well written piece and just go for the jugular here.
- "Intellisense Powershell" lets administrators auto-complete in PowerShell. Big benefit, must-have, and has been readily available to bash and zsh users in Linux-based operating systems for a long long time. Seriously - get real! Microsoft has only just started down the road of a headless server OS path where automation can truly scale out operations and they have a lot of ground to cover. This is our first example of the Microserfs pulling their heads out of <the ground> and look at what's going on outside their <world>.
- "Live Migration" lets Hyper-V guests be moved without disruption to new hosts. I can't honestly say I'd touch Hyper-V without some sort of hazmat suit on. Seriously, this is a "new" feature for Microsoft? VMware vmotion has be been doing this for vmware customers for a while. Yes, Hyper-V is free and VMware is paid, but with Hyper-V you're not getting your money's worth. Maybe if VMware doesn't innovate at all for the next decade, Hyper-V will catch up enough to make it a viable option for anything other than a test lab or party tricks.
- "NIC Teaming" oddly I don't consider a "must-have", but this would be a feature possibly 20 years or more behind the times. Hardware independent NIC teaming for bandwidth agreggation and fault tolerance has been the norm on any network operating system outside Microsoft Windows since, well, forever. Where MS admins have historically depended on NIC vendors' drivers to provide this functionality to date, there at least is a path to do this in Winblows so though this is an important feature, I wouldn't buy Windows Server 8 specfically for it.
- "Claim Definitions" is a feature that allows sensitive files to be tagged as confidential, for example and access can be based on these "claim definitions". I have no gripe here - sounds like "access control lists" based on tags. I'd like to see how flexible this functionality is but even as-is can be an important tool under Windows 8.
- "Flexible Deployment" means that you can install Windows Server "core" (the stupid headed "headless" install we know from Windows Server 2008) and then, wait for this shocker.... Upgrade to full at a later time. #facepalm I mean, seriously? You've got Ubuntu users who do in-place one-click upgrades across major versions, RedHat Enterprise Linux admins who will generally install headlessly just to get a box up and then add in all the features including the GUI in their default software package manager tool, but Windows Server users are only now going to be able to add the full Windows install into core without reformatting? Maybe with Windows 25 in the year 2050, Microsoft will shock and amaze us by letting their users get software updates for their application all from a common update utility rather getting random prompts every other day to update all the plethora of third-party applications and utilities they have installed just to make their computer usable (actually, this will never happen...).